The present invention relates to a magnetoresistive head mounted in a magnetic recording system. Particularly, it relates to a magnetoresistive head and a manufacturing method thereof by which information recorded in a magnetic medium is reproduced.
At the present time, a GMR (Giant Magneto-Resistive) head using a spin-valve, which has a basic structure consisting of a ferromagnetic layer/nonmagnetic conductive layer/ferromagnetic layer/antiferromagnetic layer disclosed in JP-A No. 358310/1992, is generally adopted for a magnetoresistive head mounted in a magnetic recording system as a read sensor. In a spin-valve, a ferromagnetic layer in which the magnetization direction is fixed in one direction by a magnetic exchange coupling with an antiferromagnetic layer is called a pinned layer, and another ferromagnetic layer is called a free layer because the magnetization direction can be freely changed according to an external magnetic field.
A GMR head using a spin-valve outputs a magnetic signal as a voltage change or a current change by using a phenomenon in which the electrical resistivity changes in proportion to the angle made by the magnetizations of the pinned layer and the free layer. Therefore, fixing the magnetization direction of the pinned layer unidirectionally (concretely, a direction perpendicular to the magnetic medium, hereinafter, it is written as “sensor height direction”) becomes most important in making the spin-valve function as a magnetic sensor. That is, it is necessary that the magnetic field needed to reverse the magnetization of the pinned layer (which corresponds to the magnetic exchange coupling field applied by the antiferromagnetic layer) is made sufficiently greater than a signal field from the magnetic medium and a leakage field from the write head, etc. Moreover, concerning the thermal history while manufacturing the magnetoresistive head and the operation environmental temperature of head, etc., the thermal stability of the magnetic exchange coupling field applied from the antiferromagnetic film to the pinned layer becomes an important factor.
At the present time, in mainstream use for the antiferromagnetic layer is an alloy expressed by Mn-M1 (where M1 is a noble metal such as Pt, etc.) including Mn of about 50 at % for the reason that a large magnetic exchange coupling field can be obtained and the thermal stability is excellent. These materials do not apply a magnetic exchange coupling field to the pinned layer as deposited on the spin-valve. Because of this, a Mn-M1 alloy is a disordered alloy having an fcc structure right after film deposition and it does not exhibit antiferromagnetism. In order to apply a magnetic exchange coupling field to the pinned layer, it is generally necessary to perform annealing in a magnetic field. It is known that a Mn-M1 alloy phase transforms into an ordered alloy having a Cu—Au I-type structure and exhibits antiferromagnetism by performing annealing at a temperature around from 230 to 270° C. Moreover, in the case when this annealing is carried out in a magnetic field, the pinned layer is magnetically exchange coupled unidirectionally with the antiferromagnetic layer, and the magnetization direction can be fixed. That is, in the case when a Mn-M1 alloy is used for the antiferromagnetic layer, an in-field annealing process is necessary not only to give a large magnetic exchange coupling field and excellent thermal stability to the pinned layer, but also to fix the magnetization direction to the pinned layer.
On the other hand, in order to obtain symmetrical response properties against the code of the signal field, it is necessary that the magnetization of the free layer is directed toward the track width direction under the condition that the external magnetic field is zero. Moreover, in order to obtain less noise and excellent linear response properties, it is necessary to apply a longitudinal biasing field to the free layer in the direction along the track width so that the free layer has a single magnetic domain structure. As a means for applying the longitudinal biasing field, JP-A No. 57223/1995 discloses a means for making a single magnetic domain in which a hard magnetic material or a laminated layer of ferromagnetic layer and antiferromagnetic layer is placed on both ends of the spin-valve and a longitudinal biasing field is applied to the free layer. Particularly, the former one is called a hard bias structure and it has become the mainstream of current GMR head structures. A hard bias structure makes the free layer single magnetic domain, so that it is effective in suppressing noise. However, in the case when the longitudinal biasing field is too high, the reproducing output becomes smaller. On the other hand, in the case when the longitudinal biasing field is too low, the problem arises that a single magnetic domain effect cannot be sufficiently obtained. Therefore, excellent reproducing properties cannot be obtained unless the magnitude of the longitudinal biasing field is optimized.
In a hard bias structure, controlling the magnitude of the longitudinal biasing field is extremely difficult because it is being decided while many factors are influencing each other in a complicatedly way, which are (1) the magnetic moment ratio of the free layer and the hard magnetic film, (2) the etching shape of the spin-valve at the track edges, (3) the geometric arrangement of the hard magnetic film in relation to the free layer, and so on. Moreover, because the hard magnetic film is so arranged, a so-called “side-reading” problem arises since the shield to shield spacing between the upper and lower shields at the track edges becomes larger than at the mid-position of the track, whereby, the effective magnetic track width does not become narrow even if the geometric track width is made narrower, and a further increase in the recording density cannot be achieved. It is clear that these problems become noticeable with narrowing the track width.
As another means of applying the longitudinal biasing field to the free layer, JP-A No. 250205/2001 discloses a means for depositing an in-stack bias film consisting of a laminated film of a bias antiferromagnetic layer/bias ferromagnetic layer/bias antiferromagnetic layer connected to the free layer after depositing a spin-valve consisting of an antiferromagnetic layer/nonmagnetic conductive layer/free layer. In this configuration, the magnetization direction of the bias ferromagnetic layer is fixed by magnetic exchange coupling with the bias antiferromagnetic layer in the direction along the track width. Moreover, a longitudinal biasing field can be applied effectively to the free layer by coupling the free layer with the bias ferromagnetic layer ferromagnetically or antiferromagnetically through the bias nonmagnetic layer (hereinafter, this means for applying a longitudinal biasing field is called an “in-stack bias structure”). In this case, an advantage is expected that the magnitude of the longitudinal biasing field can be easily controlled by controlling the thickness of the bias nonmagnetic layer.
However, in this configuration, it is very difficult to control the magnetizations of the pinned layer and the bias ferromagnetic layer in the direction along the sensor height and track width direction, respectively. That is, because the first in-field annealing process to fix the magnetization of the pinned layer in the direction along the sensor height and the second in-field annealing process to fix the magnetization of the bias ferromagnetic layer in the direction along the track width are needed, a problem arises that the in-field annealing processes affect each other and the magnetization directions of the pinned layer and of the bias ferromagnetic layer are shifted from their desired directions. JP-A No. 367124/2002 discloses that the above-mentioned magnetization direction control is carried out by using antiferromagnetic layers having different blocking temperatures and by performing in-field annealing applied in the direction along the sensor height and in the direction along the track width at different temperatures. However, in this method, it is necessary to trade off the magnitude of the magnetic exchange coupling field and the thermal stability given to either of the two above-mentioned ferromagnetic layers. In this case, there is a possibility that it becomes very difficult to obtain reproducing properties with excellent reliability.
JP-A No. 160640/2001 discloses that 90-degree magnetic interlayer coupling is used for the above-mentioned magnetic direction control method. In this specification, the interlayer interaction, in which adjoining ferromagnetic layers are directed in nearly orthogonal directions to each other through the magnetic separate layer, is called “90-degree magnetic interlayer coupling”. This disclosure example has a laminated structure of first antiferromagnetic layer/pinned layer/nonmagnetic conductive layer/free layer/90-degree magnetic interlayer coupling layer/bias ferromagnetic layer/second antiferromagnetic layer, and the magnetization directions of the pinned layer and the bias ferromagnetic layer are both fixed in the direction along the sensor height. That is, the annealing to fix the magnetization direction needs only be carried out while applying a magnetic field in the direction along the sensor height. In this case, the magnetization of the free layer can be automatically directed in the direction along the track width because an interlayer interaction is working where the magnetizations of the free layer and the bias ferromagnetic layer are directed in orthogonal directions to each other through the 90-degree magnetic interlayer coupling layer. However, in this configuration, there is a concern that magnetic poles are created at the track edges of the free layer, and making a single magnetic domain is prevented because of the influence of a demagnetizing field.